Content Snippet that bundles the current page's stylesheets and assets into a zip file.

content-snippet.js

(async()=>{
  const customCss = `
  #title-heading {clear:left}
  `;
  
  const asyncArray = (iter, map) => Promise.all(Array.from(iter, map));
  const css = (await asyncArray(
      document.querySelectorAll('link[rel="stylesheet"][href^="/"]'),
      async n => (await fetch(n.href, {mode: 'no-cors'})).text()
  )).join("\n").replace(/\/\*.*?\*\//gs, '');

  const paths = new Map();
  const pattern = /\burl\((['"]?)(?<url>\/.*?)(?<hash>#.*?)?\1\)/g;
  const rewritten = css.replace(pattern, (str, quote, url, hash = '') => {
    const name = url.split('?', 2)[0].split('/').at(-1);
    if(!paths.has(url)) paths.set(url, `./assets/file-${paths.size}-${name}`);
    return `url("${paths.get(url)}${hash}")`;
  });

  console.log(`Fetching ${paths.size} files ...`);
  const files = Object.fromEntries([
    ['./site.css', new TextEncoder().encode(rewritten + customCss)],
    ...await asyncArray(
      paths,
      async ([url, local]) => [local, new Uint8Array(await (await fetch(url)).arrayBuffer())]
    )
  ]);
  
  // Note: CSPs may prevent the module from loading.
  const {zipSync} = await import('https://cdn.jsdelivr.net/npm/[email protected]/+esm');
  console.log('Generating archive ...');
  const archive = zipSync(files, {level: 9});
  const a = document.createElement('a');
  a.download = 'styles.zip';
  a.href = URL.createObjectURL(new Blob([archive]));
  a.click();
})()

@Joshua-Beatty-Pearagon, your code does not work. @mootari 's code works. Thanks all!